Technical Field
This disclosure relates to precision metrology, and more particularly to machine vision inspection systems in which a variable focal length lens may be utilized to periodically modulate a focus position.
Description of the Related Art
Precision machine vision inspection systems (or “vision systems” for short) may be used for precise measurements of objects and to inspect other object characteristics. Such systems may include a computer, camera, optical system, and a stage that moves to allow workpiece traversal. One exemplary system, characterized as a general-purpose “off-line” precision vision system, is the QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, which is hereby incorporated by reference in its entirety. This type of system uses a microscope-type optical system and moves the stage to provide inspection images of small or large workpieces at various magnifications.
Such machine vision inspection systems are generally programmable to provide automated inspection. The machine control instructions including the specific inspection event sequence—i.e., sequential image acquisition settings (e.g., position, lighting, magnification, etc.) and how to analyze/inspect each image (e.g., using one or more video tools) are stored as a “part program” or “workpiece program” that is specific to the particular workpiece configuration.
Video tools (or “tools” for short) and other graphical user interface (GUI) features allow operations and programming by “non-expert” operators. Such tools may be used manually in “manual mode”, and/or their parameters and operation can also be recorded during learn mode, in order to create part programs. Video tools may include, for example, edge-/boundary-detection tools, autofocus tools, shape- or pattern-matching tools, dimension-measuring tools, and the like. U.S. Pat. No. 6,542,180, which is hereby incorporated herein by reference in its entirety, teaches a vision system including the use of video tools for inspection programming.
In some applications, it is desirable to operate an imaging system of a machine vision inspection system to collect an image with an extended depth of field (EDOF), that is larger than that provided by the optical imaging system at a single focus position. Various methods are known for collecting an EDOF image. One method collects an image “stack,” consisting of a plurality of congruent or aligned images focused at different distances throughout a focus range. A composite image is constructed, wherein each portion of the field of view is extracted from the particular image that shows that portion with the best focus. However, this method is relatively slow. As another example, Nagahara et al. (“Flexible Depth of Field Photography”, Proceedings of the European Conference on Computer Vision, October 2008) discloses a method wherein a single image is exposed along a plurality of focus distances during its exposure time. This image is relatively blurry, but it is deconvolved using a known or predetermined blur kernel to obtain a relatively clear EDOF image. In the disclosed method, the focal distance is altered by translating the detector along an optical axis of an imaging system. This method is also relatively slow and mechanically complex. An improved method for providing images with features at different focus distances is desirable.